1. Field of the Invention
The present invention relates to a communication apparatus like a facsimile machine, which implements communication according to the protocols that are defined by Recommendations T.30, V.8 and V.34 of the ITU-T (International Telecommunication Union for Telecommunications).
2. Description of the Related Art
Recommendation T.30 of the ITU-T defines a protocol for transmitting text via facsimile over a public switched telephone network (PSTN), Recommendation V.8 specifies a session start protocol for data transmission over the PSTN, and Recommendation V.34 defines the specifications of a modem which operates at a data signal rate of up to 33600 bps that is used in the PSTN and a 2-wire, point-to-point telephone-type private line.
FIG. 10 is a diagram showing a sequence of a protocol signal in a communication apparatus according to prior art, which communicates with a destination communication apparatus according to the protocols of Recommendations T.30, V.8 and V.34 of the ITU-T. After a line connection between a calling station and a called station is established, a digital identification signal (DIS) with bit number 6 set, i.e., bit number 6=1, is sent from the called station, and the calling station which receives the DIS without receiving a modulated answer tone signal (ANSam signal) sends a calling indication signal (CI signal). Accordingly, communication protocols according to Recommendations V.8 and V.34 are executed.
The communication protocols according to Recommendations V.8 and V.34 will be discussed. First, the called station sends an ANSam signal. Upon reception of the ANSam signal, the calling station sends a calling menu signal (CM signal). In response to the CM signal received during transmission of the ANSam signal, the called station sends a common menu signal (JM signal). A modulation mode between the calling station and the called station is determined by exchanging the CM signal and the JM signal. Upon reception of the JM signal, the calling station sends a CJ (CM end) signal, and then carries out the communication protocols according to Recommendations V.8 and V.34. Upon reception of the CJ signal, the called station stops sending the JM signal, after which the called station carries out the protocols according to Recommendations V.8 and V.34.
For example, Japanese Unexamined Patent Publication JP-A 8-214136 (1996) discloses a technique of effectively using the functions provided by the V.8 protocol. Specifically, when a calling station cannot receive the JM signal from a called station while transmitting the CM signal, the calling station switches the communication system to a half duplex communication system to receive the JM signal after completing the transmission of the CM signal. When a called station cannot receive the CM signal and CJ signal while transmitting the JM signal, the called station switches the communication system to the half duplex communication system to receive the CJ signal after completing the transmission of the JM signal.
Japanese Unexamined Patent Publication JP-A 8-298579 (1996) discloses such a technique that when communication has started in the full duplex communication, a called station, in order to allow a calling station to communicate, even if the calling station does not have a full duplex communication function, sends a DIS in the case where the called station does not detect a CM signal after sending an ANSam signal, and the calling station sends a DCS signal in response to the DIS.
In the above case where the calling station receives the DIS with bit number 6 set without receiving the ANSam signal and sends the CI signal to execute the communication protocols according to Recommendations V.8 and V.34, a communication error occurs when the calling station does not receive the ANSam signal sent from the called station that has received the CI signal, as shown in FIG. 11. This disables transmission and reception of information which should be communicated. In the case where the calling station receives the ANSam signal from the called station when the calling station has sent the CI signal for a predetermined number of times, as shown in FIG. 12, the time required for exchanging a protocol signal becomes longer, resulting in a higher communication cost.
Further, according to the prior art communication apparatus, when a called station which has sent the ANSam signal does not receive the CM signal from a calling station within a predetermined period after transmission of the ANSam signal, the called station sends the DIS with bit number 6 set, as shown in FIGS. 11 and 12. Upon reception of the DIS, the calling station acknowledges that bit number 6 is set, and sends the CI signal and then the CM signal. In this manner, the communication protocols according to Recommendations V.8 and V.34 are implemented.
When the called station which has sent the DIS can not receive the CI signal and the CM signal, however, the called station should send the DIS repeatedly. This elongates the communication time, resulting in a higher communication cost, or may result in a communication error when the protocol of phase 1 (network interaction) cannot end within a predetermined period. Further, the protocol of phase 2 (probing) or the protocol of phase 3 (training with a primary channel equalizer) may not be able to end properly, requiring that the phase be retried repeatedly. This elongates the communication time, or may result in a communication error when the phase 2 or the phase 3 can not end within a predetermined period.